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Topic: 175/200 Crankshaft Project (Read 12168 times)

I said I'd start a thread on the 175/200 crankshaft rebuild upgrade for my SR175 racer. The cranks are a bit of a weak point when you push them beyond their standard designs. The first crank I broke was in 2004 at Sandia NM which is basically outside of Albuquerque. I think I've broken 3 or 4 over the years. That first one was a standard SR175 crank. I had a spare so after I determined it was broken I switched it out at the track. Unfortunately after I got it together for the next days races, I got a little over enthusiastic trying to make up for lost time and crashed it. Needless to say that crank got ruined because I cleaned it as best as I could without taking the motor apart using all the carb and brake cleaner. Miracle it ran but running dirt thru the motor doesn't due wonders for anything. Even the Nikasil plating on the cylinders looked beat.

I don't recall when the second crank broke but by this time I had spoken to Karl Swartout (Old Swartout) and he had confirmed his broken cranks. He had some center pins made that were stepped on both sides. I sent him a couple of cranks (the broken SR175 and another I think NOS crank that I broke) and he had the broken portion turned out and bored to accept the new pin. The Bridgestone 175/200 center crank webs have a hole in one side and the center pin is part of the other half. I put the SR175 crank back together and put the other back into my racer. I eventually broke the pin in this one. I was going to use the SR175 with the new pin but the crank wasn't pressed together properly and it was too tight (or too wide?). I had purchased another NOS crank from a friend. I think I used this one as a last resort a few seasons ago only to ruin it the first season I ran aluminum rotary valve covers. The covers weren't coated and the disc rubbed enough material off to destroy all the bearings (still ran though!).

The pics are of the SR175 crank broken in the pits. The second is after I crashed the next day. The third is the pin that broke that Old Swartout had made. I'll update this a little later with where I'm at now and what I plan as a fix.

It ran good enough too finish a couple of races! I was trying to compare it to a pretty fast CT1 Yamaha and lost where I was on the track. Got lucky when I hit some soft dirt off the track! My wife was timing me and saw a plume of dust in the distance. She got nervous when I didn't come around where she was timing.

Hey Paul. So I guess it didn't "inhale". Always good to send up a distress signal (cloud of dust) when the guy with the bucket finds the fan!Enjoying this thread (maybe too much), so I'll let you get back to the tech part. Thanks for posting....bert

Is this a result of running at a higher rpm than the factory intended or is it the constant up and down torque demands of racing around a track?

Dan

The failures are always rotating bending, not torsional, starting at the radius of the step in the center shaft. I suspect the crankshafts go into a sine wave resonance mode when they get above 10,000 RPM, flexing the center shaft (they seldom break if you keep them below 10,000 RPM). Of course, the higher torque/horsepower Paul is getting just adds to the stress.

I think Paul is working to add two center bearings to better support and stiffen the center, which should push any resonance to a higher RPM, plus he's been working on using a straight center shaft with no step, which would eliminate the stress riser where the break starts. He'll have more details as he works them out.

Hey Paul. So I guess it didn't "inhale". Always good to send up a distress signal (cloud of dust) when the guy with the bucket finds the fan!Enjoying this thread (maybe too much), so I'll let you get back to the tech part. Thanks for posting....bert

What are your options for addressing this weak spot on the 175 cranks? I know very little about modifications for racing applications but am very curious on what could be done to modify a stock crank.

Is this a result of running at a higher rpm than the factory intended or is it the constant up and down torque demands of racing around a track?

Dan

Hi Dan,

I have a couple of options. Not quite sure if they will work but at least have to give them a try. The first is to remove the center pin that connects the two center webs and where the center bearing and seal ride. The original setup has that shaft step from 20mm to 16mm where they press together. I plan on removing the shaft and boring both center holes to 20mm. I've found some hardened dowels that are 20 x80mm. This will replace the original shaft. I'll detail more in posts to come.

First pic is of crank showing where they typically break. Second is of the crank I already had this done to but using the pin Old Swartout had made. That pin ultimately broke in the same fashion.

Karl is the engineer. I can't say exactly what happens. The bikes were not designed to rev much past 9-9500 rpm in my opinion. For a street bike that's pretty good. I can only attribute the breakage to anytime the bike is not in a normal type of tune. It seems that every time either the ignition or carburation causes the bike to run poorly I've broken a crank when pushed. I assume the "lugging" of one cylinder over the other causes stress at the weak point or in this case the step. Just like snapping your fingers. That quick and simple. Funny thing is the bike will remain running on one cylinder. I think I did break one crank when I was just messing with the bike on the stand. I didn't really think it rev'd to 14K. I was testing different hand held tachs against the Scitsu tach I run. I held it open a couple times too many and snap! This is the only time the bike seemed "in tune".

The 175/200's have a terrible transmission for racing as far as ratio goes. Vince might not have the problems as he has a transmission that's more "geared" for racing.

My current setup has peaky pipes. Doesn't do much under 8K. Pulls like a freight train from 10-12k and will rev to almost 13.5-14K. not too ideal for crank life.

Any step in the center pin will cause a stress riser/concentration and will lead to this kind of a failure when pushed beyond design limits. The idea of using a straight 20mm pin will eliminate that stress focus, but as you know that will just expose the next weak link. Hopefully it will occur above the desired RPM range....bert

I spent last Saturday going thru some of the various cranks that I have already pressed apart for some reason or another. In attempting to remedy the fact that I don't have any suitable cranks that are ready to go, there are many operations involved. The stock connecting rods are tough as nails but have a "heavy" big end bearing. Not to mention the pin size is 19mm, which is only common with some Suzuki's and I think a rare Kawasaki. My first crank to build should be considered the "simplest". I plan to use a straight 20mm center pin. I plan on using the stock main bearings. I won't have to modify any cases and can use the ones I already run. since the rods are scarce, I plan on using Yamaha KT100/RD125 rods. Almost the exact dimensions as the Bridgetsone rods except they have a 20mm big end pin. So the holes for the connecting rods will be opened to 20mm along with the center holes for a straight pin. The Bridgestone rods do not use shims or washers on the big end. The Yamaha rods do. Some of the cranks that either broke or had some damage from the rod edge no longer have a nice smooth surface for the rod. Vince removes 1mm from each crank cheek because the rod he uses is 2mm wider than the Bridgestone rod. But it has a 19mm pin so that much easier to fit. I want to try using the washers so I will remove 1mm for each web to account for the washer and bore the holes to 20mm. I have found some "light" big end bearings that take loose rollers. The cages are silver coated and I think there are 14 rollers as opposed to 11 on the stock bearing. The rollers are also wider than the stock ones. This should hopefully "fix" the galling that happens from the portion of the cage that drags on the inner face of the rod after much abuse.

More to come. Pics of various crank pieces that I marked so I can relate to the machinist what operations will be performed. Bottom pic is of a couple Bridgestone rods.

Karl - awesome explanation and I think I understand but I have to admit, I think a lot of that went over my head. Not your fault.

Paul, please keep the updates coming. This fascinates me to no end. Although I am not a racer and most of this is territory I will never venture into, I really am interested in your progress and solutions.

Top pic is some samples of the Yamaha KT100 rods. These are available in a light, medium and heavy. From my understanding the rods can stretch under extreme conditions. Some karts are capable of rev'n to almost 20K. I have a set of the light rods that I will use first along with the bearings and washers (actually for a Honda CR85). If you compare a stock Bridgestone rod to one of the heavy rods, they are an almost exact match. Makes me think TKR which I believe made the original BS rods and is now TKRJ make the Yamaha rods.

The second pic is of two center webs. I had the rusty one rough bored (big Drill bit!) to get an idea of how close the pins will end up. The hole is just shy of 20mm. I didn't open the connecting rod hole at the time though. Not much difference between 19 and 20mm.

The third pic is of the pins I plan to try. They are industrial metric dowel pins. I was just going to use them to mock up the crank prior to having some special pins made. When I showed them to my machinist, he suggested to give them a try. He said he can shorten them and put centers in the ends for truing after they're pressed together. We'll see. Nice thing is they are cheap compared to having special pins made.

I was trying to find thinner washers/shims for the big end. Some Vespa scooters use a 20mm pin and have washers that are 0.5mm think. stock ones are plain copper but there are some for racing applications that are silver coated. Can't seem to find them separately. The less I have to take off the inner cheeks would be nice but I will probably just use the 1.0mm washers for now.

I have to sit down and write up all the operations that I need the machinist to do to each crank. I have the taper on the outside crank webs (the part you see thru the port opening) cut so as to have a smoother flow with no abrupt shoulder. When you open the port in the case, there is then a shoulder at the bottom edge. Most tuning books would have you build up the bottom wall of the cases with an epoxy but I would not want it to ever come loose! I've had a couple of cranks cut this way and I need a few more cut.

I stamped every crank with a set of numbers so I can write down what operations need to be done to each crank ie cut outer web P1 taper, bore P1 outer connecting rod hole to 20mm, bore P1 inner center holes to 20mm, remove 1mm material from P1 inner connecting rod face etc. I plan just starting with two and see how it goes.

I will probably have a fixture made to press the center halves together so they are 180 deg apart. I have seen some examples (lots for singles not many for twins) online but can't remember where I saw them. Punch press die sets are easy to find and modify. These are used for making all kinds of parts. They have big thick top and bottom plates and have ground guide pins to keep them aligned. I have a friend who has a Suzuki X-6 and the inner webs are slightly larger than the Bridgestones. My idea was to bore the top and bottoms of the press plates to accept a round die, say 5-6in in diameter. I would have the die machined to fit the Bridgestone inner web. Another die could be made to fit the Suzuki web. This way the dies could be interchanged to the same press plate. Still have a few things to figure out on that but just have to sit down and do that too.

Good work,Paul,you're certainly getting there with these cranks.I've just rebuilt the current crankshaft and this one runs Yamaha rods.I use the Samarin silver plated cages..with Samarin pins and silver plated thrust washers with the crank cheeks machined to take them as you describe.The rods are stock generic Yamaha 125/200.We went the other way with crankpins...we run the 20mm pin with the ends (Fitting into crank cheeks) ground down to 19mm.I think the KT100 bearing is a nice option..along with the thrust washers.

This crank failed at Anglesea in June though to be fair i was destruction testing it...get this:It had done six seasons of racing without fault.This ran a 20mm straight pin between cranks and was also running a ball bearing center main..as opposed to the standard roller.Does the ball allow a little more whip that helps with the snapping issue? Not sure.One thing that is for sure..i can't see you snapping a straight pin..the shoulder is,in my view,the killer.

Ever thought,while it's apart,of indexing the pin and crank wheels at 180 degrees with keyways? Quite a simple task for a decent machinist and guarantees future alignment.Maybe a woodruff keyway cut at the 3 o'clock and 9 o'clock positions in the wheels with the crankpins at tdc ..or even both at the 6 o'clock position?

I think both you and Paul have good solutions. The straight center pin certainly will help and I think Paul's trial of a stock off-the-shelf pin will probably be sufficient. Commercial dowels are pretty tough material, and the original BS crank is relatively soft material, which contributes to the failures. It sounds as if you've proven a straight pin is the answer.

I think it's great that you guys have found rods and bearings that can be used, especially the silver plated bearings. In my opinion, that's a weak link in the connecting rod area as the wear of the rod bore from the cage OD causes more roller load transferred to the cage bars, which will eventually cause the cage to break up. I saw some of that in my previous life as a transmission engineer dealing with planetary gearsets. Silver plating is a significant improvement, although the cage design details are important too.

I have thought about using stepped pins for the connecting rod pins (well at the insistence of the guy who usually builds my cranks). He thinks it would be easier to press together if the pins were stepped so they won't "push thru". I'm not convinced but will try a couple cranks with 20mm holes. I've thought about indexing the center pins like a Kawasaki triple with key ways also.

I think I may have exceeded the max on photos. I can't seem to post anymore. I'll try and figure it out.

I have thought about using stepped pins for the connecting rod pins (well at the insistence of the guy who usually builds my cranks). He thinks it would be easier to press together if the pins were stepped so they won't "push thru". I'm not convinced but will try a couple cranks with 20mm holes. I've thought about indexing the center pins like a Kawasaki triple with key ways also.

I think I may have exceeded the max on photos. I can't seem to post anymore. I'll try and figure it out.

paul

The correct space between crankshaft flywheels on each side is 7.10 mm (.279 in.). I was lucky enough to find a couple of parallel plates that exact thickness that I use to space the the crankshaft when pressing them together, but it wouldn't be any problem to grind off a couple 8 mm nuts to the correct thickness to use as stops between the flywheels.

I've only ever had one connecting rod pin push thru, now I just put a very thin washer or shim under the end against the support plate and that solves it. Actually, the inner end needs to be nearly flush or the outer end will protrude.

Putting a keyway in the center pin will cause a stress concentration, just what you are trying to avoid.

This is a tool that I was making to check shift forks for straightness. It gave me part of the idea for making dies that could be interchanged in a jig to press to centers together on different cranks. I dropped two cranks off last week at the machinists. Hopefully he can make sense of all the operations that need to be done. -Center holes opened to 20mm (19.96/7mm) on both inner webs (remove broken center pin).-All connecting rod holes opened to 20mm (19.96/7mm) -Remove 1mm (approx) from each web for thrust washer-Bring angle cut on the outer webs deeper to connecting rod hole for smoother flow from intake port-Shorten new center pin to -2mm of orig length of standard crank center pin. Allows for thrust washers-Add centers to new pin to allow for checking true

Next will be onto the press tool and maybe the new ignition.

I ordered more of the big end bearings. One other option for rods is to have some stock rods resized for the same big end bearing. I spoke with a different machinist and he said he could probably do it. I have a pile of rods that got galled on the big end. It would be nice to recycle some.

I picked up the first cranks parts from the machinist today. They look pretty good. He was able to shorten the new center pins and add locating centers. He opened the connecting rod holes to 20mm. He removed the broken center pin and bored both center webs to 20mm. He removed the material for the shims and cut the outside taper. The only thing I forgot to specify was to put a chamfer on some of the holes he opened up. Some machinists need it all spelled out and some either question or know what to do. This is the first thing I've had this machinist do. He's a retired aero space guy and probably doesn't just do it unless it's spelled out. Shouldn't be too much of a problem. I'll get some pics when it clears up (yes, it does rain in So Cal now and then). I'll start working on the assembly jig soon!

The first pic is of the new pin and washer that I plan on using. The washer is for a Honda CR80 or 85. The pin fits a Suzuki T20 or TC250. The second pic is of the fixture the machinist made to hold the webs. He made it so any of the webs can be held to bore the holes for the connecting rod or new center hole. The material can also be removed for the washer or shim. Pretty slick.